Low noise amplifiers (LNAs) are used in many systems where low-level signals must be sensed and amplified. For example, LNAs are utilized in ultrasound imaging equipment to amplify the reflected signal sensed by an ultrasound sensor, and in radio receivers to amplify the radio frequency (RF) signal received by the antenna.
Some prior art LNAs utilize termination resistors as shown in FIG. 1. The value of the termination resistor RT is typically made equal to ZG which represents the output impedance of signal generator 10. We can assume amplifier 14 has an infinite input impedance and is presumed to be noise-free. A problem with the circuit of FIG. 1 is the inherent noise penalty due to the noise of the resistor RT.
FIG. 2 is a schematic diagram of a prior art LNA that uses active matching rather than termination to utilize the full available power of the input signal. That is, both the voltage across the termination impedance and the current into that impedance contribute to the final output signal. In the circuit of FIG. 2, the input impedance ZIN seen looking into the amplifier is made equal to the source impedance ZG by setting the value or RF equal to (1+A)ZG where xe2x88x92A is the gain of the amplifier 14, which is assumed for the moment to be noiseless and to have an infinite input impedance. The noise factor NF of the circuit of FIG. 2 is given by                     NF        =                                            2              +              A                                      1              +              A                                                          (                  Eq          .                      xe2x80x83                    ⁢          1                )            
Thus, NFxe2x86x920 dB as Axe2x86x92∞, and the circuit of FIG. 2 has lower input-referred noise than the circuit of FIG. 1. Using shunt feedback resistor RF for active matching makes the input-referred feedback resistance appear as though it is transformed to the input as a termination resistor, but with a much smaller noise penalty than that associated with the termination resistor RT shown in FIG. 1.
FIG. 3 is a schematic diagram of a practical, one-transistor realization of an LNA utilizing the input matching technique of FIG. 2. The circuit of FIG. 3 includes an NPN bipolar junction transistor (BJT) Q1 configured in common-emitter mode, a current source 16, which generates a current IC, and a feedback resistor RF. With a correct choice of IC,RF and load impedance ZL, the input impedance ZIN seen looking into the LNA can be made equal to the output impedance ZG of the signal generator.
However, while the circuit of FIG. 3 has very low noise, it also has a limited dynamic range. The dynamic range is bounded by the noise floor at the low end and by distortion at the high end. If the input signal level is lower then the noise floor, it is overwhelmed by the noise, and the LNA produces no useful output. The noise floor for the circuit of FIG. 3 is defined in terms of noise-spectral density and is typically about 1nV/{square root over (Hz)} depending on the device characteristics and bias point. At the other extreme, transistor Q1 only provides a usefully linear output signal for input voltages that have a magnitude on the order of the thermal voltage VT, which is about 26 mV at 300K. Beyond input an level of about VT/4, the response becomes markedly nonlinear, thereby introducing distortion and intermodulation which cannot be removed.
The dynamic range of such LNAs is unacceptable for many applications. For example, in a medical ultrasound imaging system, the signal attenuation from the transmitter to the receiver can be anywhere from 0 to 100 dB depending on the distance between the transceiver head and the object being imaged, and the peak signal magnitude may be of the order of 1V peak-to-peak, much greater than can be tolerated by a conventional LNA.
The input signal range of the circuit of FIG. 3 can be extended by using the well-known technique of emitter degeneration, wherein a resistor is connected in series with the emitter of Q1. However, this also introduces noise, so while the high end of the input signal range is extended, the noise floor is also raised, with the net effect that the dynamic range is not greatly improved. Thus, it is apparent that achieving low noise and wide dynamic range are mutually conflicting goals. Many solutions have been advanced to this fundamental problem, but they all have utilized a fixed circuit topology; that is, one which is independent of the signal magnitude.